This invention relates to an electroluminescence element used as a back-lighting of a display section and a control section in an electronic device and a lighting unit having the electroluminescence element.
Recently, as the diversification of an electronic device increase, device having a back-lighting behind a liquid crystal display (LCD), a display panel or switch keys, such that the display section and the control section can be identified and controlled in darkness, has increased. An electroluminescence element (it is called an EL element hereinafter) has been used as a back-lighting.
A conventional EL element used for this purpose is described with FIG. 7 and FIG. 8. The drawings are enlarged in a direction of the thickness for clarity of its configuration.
FIG. 7 is a cross sectional view of the conventional EL element. El element 6 has a laminated structure of the following layers and is formed by printing in order named;
(a) flexible light-transmittable insulating film 1 made of polyethylene terephtalate or the like; and
(b) light-transmittable insulating front electrode layer 2, which is made of indium tin oxide (it is called ITO hereinafter), formed on the entirely underneath film 1 by a spattering process or an electron beam process;
(c) luminescent layer 3 dispersed luminous powder, which is luminescent base material such as zinc sulfide, in high dielectric resin such fluoro-contained rubber, cyan-base resin or the like, underneath front electrode layer 2,
(d) rear electrode layer 4 dispersed silver or carbon resin in epoxy resin, polyester or the like;
(e) insulating layer 5 made of epoxy resin, polyester resin or the like.
EL element 6 with configuration described above is disposed in an electronic device. When an alternating voltage is applied between front electrode layer 2 and rear electrode layer 4 from the electronic device (not shown), luminescent layer 3 in EL element 6 is actuated and EL element 23 emits light from the top of insulating film 1. This light illuminates the LCD and the display panel from the rear in the electronic device. Therefore the display section and the control section can be identified in the dark.
When illuminating both sides of the electronic device, two EL elements 6 are placed so as to be opposed each insulating layer 5 of two EL elements back to back as shown in a cross sectional view in FIG. 8. When converting color of light and illuminating with multiple-color lights, two EL elements 6 having different luminescent colors are combined.
However, in the conventional EL element described above, when illuminating both surfaces of the electronic device, entire EL element is thick and the number of parts are increased because of combining two EL elements into one. This allows the electronic device to be more expensive.
The invention provides an EL element emitting light from its both surfaces, which is thinner and inexpensive by decreasing the number of parts, and a lighting unit having it. The EL element has a laminated structure of the following layers and is formed by printing in order named;
(1) a light-transmittable insulating film;
(2) a front electrode layer;
(3) a luminescent layer made by dispersing powdery fluorescent substance into high dielectric resin; and
(4) a light-transmittable rear electrode layer made by dispersing conductive powder into light-transmittable resin.
When forming the EL element having a structure the same as described above on both sides of the insulating film, a thinner double-side-lighting EL element is provided when compared to both-sided lighting by the conventional EL element combined two about separate EL elements. Further, the EL element for multiple-color lighting from both surfaces can be provided by using luminescent layers having different luminescent colors respectively. Furthermore, in the case of using a light-transmittable insulating film and a plurality of light-transmittable front electrode layers, in addition to a first and a second colors of each luminescent layer, a third color is produced by merging the first and second colors when emitting two luminescent layers simultaneously.